Infrared gas sensor technology is well established and can give selective and quantitative gas detection for a variety of gasses having vibrational-rotational absorptions at wavelengths of between 3 and 12 microns. Existing infrared sources for gas sensors at infrared wavelengths operate at high temperatures and thereby have a number of drawbacks in intrinsic safety, wavelength range, stability and life. Furthermore, existing sources emit pulses of infrared radiation at a frequency which is limited by a maximum frequency of the order of several Hertz which is not consistent with optimum working frequencies of processing electronics and limits the type of infrared signal detector which can be used in the gas sensor.
Recently developed infrared LEDs overcome the bulk of these problems in gas sensor applications. The new infrared light emitting diodes are intermittently positively biased in order to generate a series of pulses of infrared radiation. However, the output power of these pulses is strongly dependent on temperature. Accordingly, during use of such an infrared LED in gas sensor arrangements, either the LED temperature has to be monitored and the results mathematically corrected for temperature changes or the LED temperature has to be stabilised. The normal method for stabilising optical output power with respect to temperature is the use of temperature control methods, such as the combination of thermoelectric or Peltier cooling and temperature sensors. Therefore, stabilising and/or monitoring temperature adds expense and complexity to the operation of infrared LEDs in gas detection applications.
A dynamic infrared scene projector comprising infrared light emitting diodes capable of emitting both positive and negative luminescence is discussed in International Patent Application Number PCT/GB96/02374. A gas sensor including an infrared light emitting diode which can emit both positive and negative luminescence is described in an article by C. H. Wang et al entitled “Detection of nitrogen dioxide using a room temperature operation mid-infrared InSb light emitting diode” which was published in Electronics Letters, vol. 34, No. 3 (5 Feb. 1998), pages 300-301.